It appears that they are in response to the US refusing to take them off the terrorist list, as was previously agreed to.

Seriously, taking them off the list is just a stroke of a pen, and can be reinstated with a stroke of a pen, but Bush and His Evil Minions^&trade are so busy swinging their dicks that they are convincing the North Koreans that this is all some sort of ploy to destroy them.

Fracking with paranoids for giggles when they have missiles and nukes is stupid, but for Bush and His Evil Minions^&trade, stupid is as stupid does, I guess.